Atherosclerotic occlusive disease is the primary cause of stroke, heart attack, limb loss, and death in the US and the industrialized world. Atherosclerotic plaque forms a hard layer along the wall of an artery and is comprised of calcium, cholesterol, compacted thrombus and cellular debris. As the atherosclerotic disease progresses, the blood supply intended to pass through a specific blood vessel is diminished or even prevented by the occlusive process. One of the most widely utilized methods of treating clinically significant atherosclerotic plaque is balloon angioplasty.
Balloon angioplasty is an accepted method of opening blocked or narrowed blood vessels in every vascular bed in the body. Balloon angioplasty is performed with a balloon angioplasty catheter. The balloon angioplasty catheter consists of a cigar shaped, cylindrical balloon attached to a catheter. The balloon angioplasty catheter is placed into the artery from a remote access site that is created either percutaneously or through open exposure of the artery. The catheter is passed along the inside of the blood vessel over a wire that guides the way of the catheter. The portion of the catheter with the balloon attached is placed at the location of the atherosclerotic plaque that requires treatment. The balloon is inflated to a size that is consistent with the original diameter of the artery prior to developing occlusive disease. When the balloon is inflated, the plaque is broken. Cleavage planes form within the plaque, permitting the plaque to expand in diameter with the expanding balloon. Frequently, a segment of the plaque is more resistant to dilatation than the remainder of the plaque. When this occurs, greater pressure pumped into the balloon results in full dilatation of the balloon to its intended size. The balloon is deflated and removed and the artery segment is reexamined. The process of balloon angioplasty is one of uncontrolled plaque disruption. The lumen of the blood vessel at the site of treatment is usually somewhat larger, but not always and not reliably.
Some of the cleavage planes created by fracture of the plaque with balloon angioplasty form dissection. A dissection occurs when a portion of the plaque is lifted away from the artery and is not fully adherent and may be mobile or loose. The plaque that has been disrupted by dissection protrudes into the flowstream. If the plaque lifts completely in the direction of blood flow, it may impede flow or cause acute occlusion of the blood vessel. There is evidence that dissection after balloon angioplasty must be treated to prevent occlusion and to resolve residual stenosis. There is also evidence that in some circumstances, it is better to place a metal retaining structure, such as stent to hold open the artery after angioplasty and force the dissected material back against the wall of the blood vessel to create an adequate lumen for blood flow.
Therefore, the clinical management of dissection after balloon angioplasty is currently performed primarily with stents. As illustrated in FIG. 24A, a stent is a tube having a diameter that is sized to the artery. A stent is placed into the artery at the location of a dissection to force the dissection flap against the inner wall of the blood vessel. Stents are usually made of metal alloys. They have varying degrees of flexibility, visibility, and different placement techniques. Stents are placed in every vascular bed in the body. The development of stents has significantly changed the approach to minimally invasive treatment of vascular disease, making it safer and in many cases more durable. The incidence of acute occlusion after balloon angioplasty has decreased significantly with stents.
However, stents have significant disadvantages and much research and development is being done to address these issues. Stents induce repeat narrowing of the treated blood vessel (recurrent stenosis). Recurrent stenosis is the “Achilles heel” of stenting. Depending on the location and the size of the artery, in-growth of intimal hyperplastic tissue from the vessel wall directly through the tines or openings in the stent may occur and cause failure of the vascular reconstruction by narrowing or occlusion of the stent. This may occur any time after stent placement. In many cases, the stent itself seems to incite local vessel wall reaction that causes stenosis, even in the segment of the stent that was placed over artery segments that were not particularly narrowed or diseased during the original stent procedure. This reaction of the blood vessel to the presence of the stent is likely due to the scaffolding effect of the stent. This reaction of recurrent stenosis or tissue in growth of the blood vessel is in response to the stent. This activity shows that the extensive use of metal and vessel coverage in the artery as happens with stenting is contributing to the narrowing. The recurrent stenosis is a problem because it causes failure of the stent and there is no effective treatment. Existing treatment methods that have been used for this problem include; repeat angioplasty, cutting balloon angioplasty, cryoplasty, atherectomy, and even repeat stenting. None of these methods have a high degree of long-term success.
Stents may also fracture due to material stress. Stent fracture may occur with chronic material stress and is associated with the development of recurrent stenosis at the site of stent fracture. This is a relatively new finding and it may require specialized stent designs for each application in each vascular bed. Structural integrity of stents remains a current issue for their use. Arteries that are particularly mobile, such as the lower extremity arteries and the carotid arteries, are of particular concern. The integrity of the entire stent is tested any time the vessel bends or is compressed anywhere along the stented segment. One reason why stent fractures may occur is because a longer segment of the artery has been treated than is necessary. The scaffolding effect of the stent affects the overall mechanical behavior of the artery, making the artery less flexible. Available stenting materials have limited bending cycles and are prone to failure at repeated high frequency bending sites.
Many artery segments are stented even when they do not require it, thereby exacerbating the disadvantages of stents. There are several reasons for this. Many cases require more than one stent to be placed and often several are needed. Much of the stent length is often placed over artery segments that do not need stenting and are merely adjoining an area of dissection or disease. Stents that are adjusted to the precise length of the lesion are not available. When one attempts to place multiple stents and in the segments most in need of stenting, the cost is prohibitive since installation and material is required per stent. The time it takes to do this also adds to the cost and risk of the procedure. The more length of artery that receives a stent that it does not need, the more stiffness is conferred to the artery, and the more scaffolding affect occurs. This may also help to incite the arterial reaction to the stent that causes recurrent stenosis.